President Biden on Thursday signed a new security memorandum designed to strengthen security against weapons of mass destruction (WMD) and better secure related materials to reduce the threat of radiological and nuclear terrorism.

Concerns about WMD terrorism aren’t new and have been a priority for the U.S. for over 20 years but the issues around it “are not spoken of as often or debated as thoroughly as they should be,” Elizabeth Sherwood-Randall, Biden’s homeland security adviser, said at an event hosted by the Nuclear Threat Initiative.

“The terrorist threat has evolved,” she said. “It’s become more ideologically diffuse and geographically diverse and as a result, we’re evolving our counterterrorism enterprise to ensure that it’s well positioned and nimble enough to meet emerging threats in real-time.”

The strategy is outlined in National Security Memorandum 19 and includes three lines of effort and related goals. The White House released a fact sheet with an overview of the strategy, which is classified.

Counter WMD terrorism is the first line of effort and provides a comprehensive plan for federal agencies to “prevent, mitigate, and respond to WMD terrorist attacks,” according to the fact sheet. Some of the policy goals include preventing non-state actors from acquiring WMD, improving international collaboration to counter WMD terrorism, and enhancing resilience and recovery from a WMD event.

The Department of Homeland Security monitors the nation’s land and seaports for the potential introduction of illicit nuclear materials into the country. Customs and Border Protection uses radiation portal monitors that scan cargo conveyances for these materials.

DHS also works with major urban areas around the U.S. through the Securing the Cities program to provide radiological detection equipment, related training and operational planning.

The two other lines of effort, advancing nuclear and radioactive material security, are concerned with securing material sources throughout their life-cycles, having consistent standards for their transportation security, encouraging the use of non-radioisotopic technologies where possible, only producing quantities necessary for weapons-usable materials, and avoiding producing and accumulating weapons-usable nuclear materials in new civil reactors or for other civil purposes.